1. Field of the Invention
The invention relates notably to a method and system of communications, for example in a communications network that uses a UWB type protocol and in which the transmission is carried out by pulses.
It can be applied for example to communications between mobile terminals and a central station responsible for making massive deliveries of information to these mobile terminals. The station is generally called “Hotspot” in telecommunications literature. The invention relates chiefly to “large-scale consumer” applications such as data servers in airports, railway stations and shopping centers. At the same time, it can also be deployed temporarily for action teams (such as teams of firefighters, doctors, special forces, maintenance teams in high-risk areas etc).
It can be applied in communications networks using a TDMA (Time Division Multiple Access) type of protocol. It is also used in applications that rely on a frame partially formed by a zone reserved for recurrent streams, equivalent to a TDMA frame.
2. Description of the Prior Art
For example, a WiFi type of “Hotspot” station based on the 802.11b standard enables the delivery, on a channel, of a radio bit rate of 11 Mb/s shared among all the users in applications that are essentially data transfer applications. The following are the characteristics of this station:
a typical range of about some hundreds of meters in unencumbered space;
a 2.45 GHz frequency band;
a CSMA (Carrier Sense Multiple Access) access protocol.
Access Protocols
The two protocols most commonly used in small-sized or medium-sized radio networks are the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) and the TDMA (Time Division Multiple Access) protocols. Each has advantages and drawbacks that make it suited to different users, mainly in terms of service quality and network topology.
The CSMA/CA protocol is based on random and hence non-deterministic access which means that it is inefficient for services requiring constant service quality in terms of regularity and time schedules. This protocol notably has the advantages of offering equality of access to the carrier and simplicity of implementation. However, its performance is mediocre in terms of bit rate, firstly because of the periods of time lost between the transmission of frames and, secondly, because of collisions. Furthermore, it is not possible to ensure adequate service quality for voice and video owing to the variable nature of transmission times and the fact that there are no boundaries on these transmission times.
In the case of the centralized type of TDMA protocol, the time is evenly subdivided into frames of fixed length, themselves subdivided into different zones with various purposes. A station plays the role of network coordinator in assigning the resources of each frame to the different users depending on their needs. This station is called a central station or, again, a master station. A station wishing to send must therefore always make a preliminary request to this master station, which assigns it a time resource proportional to its needs, provided that there is sufficient availability.
There are therefore no collisions to be managed in a TDMA protocol, except in the access channel to the network for which it is not possible to have any other solution but which takes up only a small part of the frame. Furthermore, with this system, it is possible to transmit regular streams such as voice or video streams which require a constant bit rate and low latency time. However, the traffic generated by the signaling may be great.
Existing UWB Systems
The physical layer of the transmission system proposed according to the invention is based notably on ultra wideband technology. The principle of UWB lies in transmitting information in very short pulses (of the order of 1 ns) in baseband. This results in two characteristic properties of UWB, namely:    1) spectral power density (Power Spectral Density) with a very wide band (>1 GHz) and thus a very low maximum PSD level comparable to noise. The system thus has high immunity against scramblers and interference sources, whether intentional or not, owing to its bandwidth and does not disturb other transmission systems present in the passband of the proposed system. This therefore intrinsically leads to low probability of interception and low probability of detection (LPI/LPD).    2) the absence of the carrier frequency which simplifies radio processing operations at reception relative to conventional receivers.
UWB (or again radio pulse) transmission systems have essentially been described at the physical layer level, in the reference [1] M. Z. Win and R. A. Scholtz, << Impulse radio: how it works>>, IEEE Com. Letters, vol. 2, no. 2, February 1998, pp. 36-38, This physical layer uses a technique of time hopping code division multiple access or TH-CDMA. The idea is to transmit the pulses by spacing them out in time pseudo-randomly, each user having his own sequence. In principle, this enables the use of the UWB in an asynchronous multi-user context that is particularly adapted to ad-hoc networks. The use of this physical layer with practical systems is described in two articles:                M. Z. Win, X. Qiu, R. A. Scholtz, and V. O. K. Li, “ATM-based TH-SSMA network for multimedia PCS”, IEEE JSAC, vol. 17, No. 5, May 1999, pp. 824-836, and        S. S. Kolenchery, J. K. Townsend, and J. A. Freebersyser, <<A novel impulse radio network for tactical military wireless communications>>, IEEE Milcom Conf., Bedford, Mass., USA, October 1998, pp. 59-65.        
These documents describe the TH-CDMA technique of access without going into detail on the MAC layer and the structure of the corresponding frames. Furthermore, since the lag between two successive pulses is set so as to prevent multiple paths (it is typically set at 100 ns in indoor conditions) and since a symbol has to be transmitted on several pulses (with integration enabling an increase in the signal-to-noise ratio), the maximum bit rates that can be reached are of the order of a few Mb/s only.
At present, the efficiency of a TDMA protocol applied to UWB is lower than that obtained with narrowband transmission methods. The cause of this low efficiency is, firstly; the lengthy period of time needed for synchronization in UWB, which may go up to several milliseconds (depending on the length of the acquisition codes and the reception structure) and secondly the fact that the specific nature of the pulse waveform is not taken into account. Indeed, the low cyclical ratio of the pulses used in UWB (about 1%) means that it is possible to make use of large zones of “silence”. Now, if a classic TDMA is used to separate two users, i.e. by making them transmit one after the other, the unused time between the pulses sent by a same user is lost. This time interval enables the passage of a pulse response following the sending of a pulse by any station whatsoever.
In the method according to the invention, for example, the pulses sent by a same user station or by several different stations are made to approach one another to the maximum extent in the direction of communication (from the central station to the users and vice versa). To this end, the invention applies a technique of precise synchronization (it is aimed to achieve 1 ns for example) known as the fine synchronization of the user stations followed by a technique of equalization of the transmission channel on the pulses at reception, in order to combat interference between successive pulses. Furthermore, in the case of centralized communications networks, the method may entail the performance of a complementary interlacing of the pulse trains either from the different users addressed to the central station or from the central station to the users.
The idea in particular is to offer a TDMA protocol optimized for the UWB physical layer, enabling high bit rates to be achieved while at the same time preserving constant quality of service.